10. SPC – Attribute Control Chart

SPC control chart determines the current or improved processes’ control  state while evaluating trend of data points, and the control chart can be defined as either quantitative or attribute control chart based on the data you collected. In this section, attribute control charts will be discussed and the following are the general types of attribute control charts with respective description.

For the appropriate chart selection for attribute control chart, please refer to the SPC Introduction Link before deciding what’s the appropriate chart.

The following information contains the chart description along with calculating the control limits.

Detailed Attribute Control Chart explanation

Detailed Attribute Control Chart Setup Steps

Detailed Attribute Control Limit Calculation

The following examples are the raw data and illustration of examples for attributive control charts.

p-chart Example:

This example shows appearance defect portion for different batches, 30 subgroups with different observations were recorded. And average failure proportion was roughly around 19.6% based on the trend above from p-chart’s observation.

Remark: Since sample size per subgroup is different, therefore control limit is drawn unevenly.

np-chart Example:

This example shows appearance defect portion for different batches, 30 subgroups with same observations (all n = 50) were recorded. And average defect count was roughly 11 pieces based on the trend above from np-chart’s observation.

U chart Example:

This example shows appearance defect portion for different batches, 30 subgroups with different observations (ranges from 110 – 130) were recorded. And average defect proportion with entire subgroups are roughly 9.6% based on the trend above from U-chart’s observation.

Remark: Since sample size per subgroup is different, therefore control limit is drawn unevenly.

c chart Example:

This example shows appearance defect portion for different batches, 30 subgroups with different observations (all sampled within the same subgroup size of 120) were recorded. And average defect count is 10.7 pcs based on the trend above from c-chart’s observation.